The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of bearing weight through a damaged, unstable vertebral joint. One conventional method of managing these problems is to remove the problematic disc and replace it with a prosthetic implant (such as a fusion body, spacer or a motion disk) within the intervertebral disc space.
U.S. Pat. No. 6,083,228 (“Michelson '228”) discloses abrading elements particularly designed for preparing the intervertebral space in the spine for reception of the implant between adjacent vertebral bodies. In one embodiment, Michelson '228 discloses a device that includes a handle, a shaft, and a mounting member at one end of the shaft. An abrading element is mounted on the mounting member and is coupled to a drive mechanism. The drive mechanism is operable to move the abrading element in at least one degree of freedom to create surfaces having predetermined contours in the endplates of the adjacent vertebral bodies. FIGS. 21–23 of Michelson '228 disclose vibratory abraders. However, Michelson '228 discloses that the abrader of FIG. 21 produces vibratory motion in a plane generally parallel to the abrading surface of the abrading element. There is no disclosure in Michelson '228 of a vibratory element that moves essentially laterally to the instrument shaft.
U.S. Pat. No. 5,387,215 (“Fisher”) is a surgical device capable of cutting and removing medium to hard body tissue such as cartilage and bone from a joint region or similarly restricted interior space within the body. The surgical instrument includes a stationary carrier sized for insertion into a joint or similar restricted interior space within the body and having a first and second end. The carrier is provided with an apical aperture at the end to be inserted into the joint. A cutter is axially disposed within the carrier and is provided with at least one cutting surface at one end that protrudes from the aperture of the carrier. The cutter is driven by the motor that moves the cutter in a linearly reciprocating fashion that abrades away tissue depending on the configuration of the cutting surface.
U.S. Pat. No. 6,159,214 (“Michelson '214”) discloses a vertebral body milling device for creating a space of selected shape and dimensions across the disk space between adjacent vertebral bodies of the spine, each of the adjacent vertebral bodies having a vertebral endplate adjacent to the disc space, comprising:    i) a milling block configured at least in part for placement across the disc space and against the outer surface of the adjacent vertebral bodies and configured to hold the adjacent vertebral bodies, and comprising:            a) a front face for placement against the vertebral bodies,        b) an opposite back face,wherein the milling block has an access aperture configured for providing access for the width of the space to be prepared to at least one of the adjacent vertebral bodies from said back face and through said front face of said milling block, and            ii) a bone removal device for removing at least a portion of bone from at least one of the vertebral endplates adjacent the disc space, said bone removal device being configured to access the vertebral endplates through the access aperture.
FIG. 15c of Michelson '214 discloses a milling block in which pin holes are formed through the front and back faces and receive anchoring pins 128 that fixedly secure the milling block to the outer surface of the vertebral bodies. The diameter of the pin hole appears to be essentially equal to that of the pin shank, so that the milling block can not move relative to the vertebral body once anchored by the anchoring pins. The relatively fixed position of the milling block vis-a-vis the vertebrae prevents the opposing vertebrae from moving vertically during insertion of instruments such as distractors or rotary drills. Accordingly, the front lips of the vertebrae are necessarily removed during endplate preparation. These lips may be important in retaining the implant within the disc space.
FIG. 19 of Michelson '214 discloses a milling block in which a bone removal device passes through the block via an entry opening 114 in the block that is wider than it is tall. The predominant width of the aperture allows the bone removal device to move transverse to its longitudinal axis so that wide portions of the vertebral endplates adjacent the disc space are milled by a relatively small diameter milling tool to create a rectangular-shaped space for receiving the implant. However, since the cutting element is moved transversely over a significant distance (i.e., at least twice the diameter of the cutting element), the prepared disc space has a substantially uniform height across that transverse axis, and not a curved height that more closely mimics the natural contours of the disc space.
Therefore, it is an object of the present invention to provide a milling block that helps the surgeon to retain the endplate lips.
It is another object of the present invention to provide a milling block that provides helps the surgeon provide a contoured disc space height across the transverse axis.